The invention relates to a method for adjusting the electrical length of a coaxial cable which has coaxial plug connectors on both cable ends. At least one of the plug connectors has an adjustable length.
Coaxial cables which have plug connectors on both ends are typically referred to as preformed cables, sometimes also as jumper cables. The specified electrical length of such preformed cables frequently has a rather tight tolerance. Although the electrical length of the preformed cable is related to its mechanical length, the predetermined nominal value of the electrical length can not be attained simply by cutting the cable to the corresponding mechanical length, even when the electrical length of the two plug connectors is taken into consideration. This is mainly due to the fact that the electrical length of a cable may vary over the mechanical length (due to small deviations of the dielectric constant and/or the characteristic impedance). Moreover, the tolerances added by cutting of the cable to a nominal length, by preparing the cable ends for installation of the plug connectors and finally the linear tolerances of the plug connectors themselves may invariably be greater than the tight tolerances required for the electrical length of the preformed cable. For this reason, the preformed cable is cut with sufficient excess length and its electrical length is adjusted by measuring the phase angle that corresponds to the electrical length with a phase measurement device, typically a vector network analyzer. This measurement is performed while changing the mechanical length of at least one of the plug connectors until the actual value of the phase angle is equal to be preset nominal value. It is known to use conventional plug connectors with an inner conductor having several sliding contacts that move telescopically inside of one another. The outer conductor is also made of several multi-part threaded sleeves that are screwed into each other. The telescopically moveable inner conductor elements are received in several insulator support members which center and guide the inner conductor of the connector. The threaded sleeves that form the outer conductor of the connector, are adjusted and secured in their final position, typically with lock nuts. The fabrication of preformed coaxial cables with a defined electrical length therefore necessitates the use of complex plug connectors having a large number of precision-machined parts which take-up space and require time-consuming manual adjustment, for example, by rotating the threaded sleeves that form a part of the outer conductor of the connector. The threaded sleeves must also be secured in the adjusted position.
It is therefore desirable to provide a simple and small plug connector with a low parts count, whose electrical length can be easily adjusted after installation on a coaxial cable.
According to one aspect of the present invention, a coaxial plug connector with an adjustable length includes an outer conductor located on the cable-side and adapted to be connected to an outer conductor of the cable. The connection-side outer conductor of the connector is received in the outer conductor located on the cable-side by a press fit and can be displaced in the axial direction by a distance (e). The inner conductor of the connector can be displaced in an insulator support member by at least the adjustment distance (e) of the connection-side outer conductor of the connector.
The plug connector includes has a low parts count and can be manufactured easily and inexpensively. The preformed cable can be shortened by controllably pressing the connection-side outer conductor of the connector into the cable-side outer conductor of the connector in an axial direction. The cable is manufactured with a sufficient positive tolerance relative to the nominal value of the electrical length.
Preferably, the cable-side outer conductor of the connector is adapted to be soldered to or crimped onto the outer conductor of the cable. These are inherently known connection methods which provide preformed cables with tight tolerances with the typically required high intermodulation separation.
Advantageously, the outer periphery of the cable-side outer conductor of the connector has a profile that is complementary to that of a thrust block. This arrangement provides the necessary axial support when the connection-side outer conductor of the connector is subsequently pressed into the cable-side outer conductor of the connector.
The outer periphery of the connection-side outer conductor of the connector can have an annular shoulder that limits its maximum adjustment distance.
Advantageously, the inner conductor of the connector can be formed in one-piece and adapted to be soldered to the inner conductor of the cable. This plug connector can be manufactured more cost-effectively than a plug connector with a conventional telescopic plug inner conductor which is supported by insulator support elements in several places.
According to another aspect of the invention, a method is provided for adjusting the electrical length of a coaxial cable with a respective coaxial plug connector on both ends. At least one of the plug connectors has a connection-side outer conductor that is received in the outer conductor located on the cable-side by a press fit and can be displaced in the axial direction by a distance (e). The inner conductor of the connector can be displaced in the insulator support member by at least the adjustment distance (e) of the connection-side outer conductor of the connector. The method includes the following steps:
connecting a plug connector with a phase measurement device
shorting the plug connector having the adjustable length with a short-circuit plug
clamping the plug connector having the adjustable length and the short-circuit plug in a pressing device having a piston acting in an axial direction on the short-circuit plug, with the cable-side outer conductor of the connector of the plug connector being supported against a thrust block of the pressing device
shortening the electrical length of the cable by operating the pressing device until the electrical length reaches a preset nominal value as measured with the phase measurement device.
According to one embodiment of the method, the pressing force is measured during the pressing operation, preferably as a function of the distance traveled by the piston. A check is performed to determine if the measurement value is greater than a minimum value and smaller than a maximum value. The pressing force can be measured either by using a press equipped with a force/distance measurement device, or by placing a force measurement device between the piston of the press and the short-circuit plug. If the measured pressing force is outside the tolerance limits defined by the minimum value and the maximum value, respectively, then the respective cable is discarded as scrap, because the plug connector may not have sufficient mechanical strength If the pressing force is too small, and the plug connector can be mechanically damaged if the pressing force is too large.
Further features and advantages of the present invention will be apparent from the following description of preferred embodiments illustrated in the drawings and from the claims.